LUCIA AND ATREVIDA GLACIERS 75
photographs, also, a distinct lowering of the ice surface is noticeable, in this case surely of more than 100 feet.
It was found possible to walk freely over the ablation moraine though, owing to its greater roughness, with much more difficulty than in 1905. Alder was no longer growing on this part of the glacier but there was a thick cover of ablation moraine, though with many bare, debris-stained slopes and others with only a thin veneer of moraine. Melting was in rapid progress and small streams were coursing down all the slopes, but there were no large moulins, for the ice drainage had evidently not yet progressed far enough to develop these as they were developed in 1905. Crevasses were seen in many places and care was necessary lest one should step into those hidden from view by the moraine. Several times such a crevasse was unexpectedly discovered when stepping on what seemed to be solid, moraine-covered ice. We soon learned that the ridges were the places to be avoided and that the depressions offered the safest routes, for few and only shallow crevasses were found in the valleys, while the ridge crests were almost uniformly the sites of crevasses. The explanation of this fact is not difficult. After the glacier was broken by the advance, the moraine slid into the newly-formed crevasses, and, when ablation proceeded to lower the glacier surface, these filled, or partially filled, crevasses, with their greater depth of moraine, were so protected that they were preserved and stood above the level of the inter-crevasse areas. Then the rock fragments on and in these elevated crevasse areas slid down into the valleys, and doubtless the next stage will be the rising of the filled valleys and the lowering of the emptied crevasse areas.
In addition to the ridges due to crevasses the ablation moraine is diversified by bands and patches of rock predominantly of a single kind, such, for example, as black shale or conglomeratic rock, and in these areas are found unusual numbers of large angular blocks. They did not seem to be arranged in any order that could be correlated with flowage lines and may possibly be interpreted as rock falls and avalanches transported down the valley and spread out irregularly by the movement of the ice and by distribution through sliding, as ablation lowers the ice and undermines ridges and fills depressions.
It would be interesting to know the rate at which ablation has been proceeding on this ice surface. That the rate must have been rapid is indicated by the evidence furnished by the Atrevida and other glaciers. This evidence is of several kinds. A comparison of photographs from the same point show a general lowering of the surface sufficient to be apparent in photographs. It is possible, as already stated, that some of this lowering may be due to flowage after the first vigorous advance, but much of it, is certainly due to ablation, for the old crevassing and the noticeable proportion of clear ice have both been in large part destroyed, and only ablation could account for this. Moreover, the transformation of crevasses to ridges 50 to 100 feet high can be explained only as a result of ablation.
The only specific knowledge that we have regarding the rate of ablation in this climate are the measurements made on Hayden Glacier in 1906 when in a period of twelve days, in late July and early August, it was found that the surface of the glacier was lowered at the rate of 4 inches a day. This, however, was on a smooth ice surface, and at a much greater elevation, where the temperature on clear days descended below freezing point even before the sun set. The surface of Childs Glacier was lowered 7 inches a day in